Well, it looks like using single user for replication is bad idea for MDB.
debug log:
slapd[23170]: do_bind: version=3 dn="cn=repmgr,ou=ldapusers,o=test1"
method=128 slapd[23170]: daemon: epoll: listen=7 active_threads=0 tvp=zero
slapd[23170]: => mdb_entry_get: ndn: "cn=repmgr,ou=ldapusers,o=test1"
slapd[23170]: daemon: epoll: listen=8 active_threads=0 tvp=zero
slapd[23170]: => mdb_entry_get: oc: "(null)", at: "(null)"
slapd[23170]: daemon: epoll: listen=9 active_threads=0 tvp=zero
after this strace show mentioned âAssertion failed'.